I have never seen this happen.

I have however experienced that a ISP will write to you because you have a faulty modem (some Huawei device) and asks you to not use it anymore.

Visit eBay and search for "blocked IMEI" or variants. There are plenty of used phones which are IMEI locked due to either: reported lost, reported stolen, failed to make payments, etc.

I the lines between IMEI banning or blacklisting and the modern unlocking techniques they use have been blurred a little bit and so some carriers and some manufacturers don't really want to do or spend time doing the IMEI stuff and would prefer to just handle it all via their own unlocking and locking mechanisms.